Democritus: The Greek philosophers wanted to explain the natural world; through observations, philosophers believed that a "primary matter" existed. It was because of this “primary matter”, changed in different ways that all other things were created. Democritus expanded the idea to state that matter was composed of small particles called "atoms" that couldn’t be divided more. These atoms were all composed of the same primary matter with the only differences between them being their size, shape and weight. These differences explained the differences in the properties of the matter around us. Democritus’ ideas were ignored for the next 2,000 years; however, they proved to be very useful in the further research of the atoms.
John Dalton: For nearly 2000 years science was unable to come up with experiments capable of testing Democritus’ ideas. During the 19th century, a huge amount of data on how substances react with each other was collected. From this data, some simple laws of chemical reactivity had been developed, including the law of conservation of matter. While others had similar theories, John Dalton is usually credited with developing the first atomic theory.
Dalton's theory:
Matter is composed of small particles called atoms.
All atoms of an element are identical, but are different from those of any other element.
During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged.
Atoms always combine in whole number multiples of each other.
In addition to helping explain the laws of chemical reactivity, Dalton's theory also helped further the concept of atomic weights. He made symbols for some elements and a table, a big step in this time for the development of the atom.
Dmitri Mendeleev: Contributions of earlier 19th century scientists led to the accurate determination of atomic weights of elements and chemical formulas of compounds made from them. These determinations gave way for the first periodic table!! Whoop whoop! Dmitri Mendeleev tried to classify the elements in an exact way. He wanted this system to be numerical which meant, at this time, atomic weight. By putting them in order of increasing atomic weight, he found similar properities and lined them vertically with each other. He found some holes in the table where some elements should exist. He made predictions about this which helped in later discoveries
Henri Becquerel then Marie and Pierre Curie:
Henri Becquerel discovered that a sample of uranium was able to expose a photographic plate even when the sample and plate were separated by black paper (http://www.neoam.cc.ok.us/~rjones/Pages/online1014/chemistry/chapter_8/pages/history_of_atom.html).
He also discovered that the exposure of the plate did not depend on the chemical state of the uranium (what uranium compound was used) and that it must be due to some property of the uranium atom (http://www.neoam.cc.ok.us/~rjones/Pages/online1014/chemistry/chapter_8/pages/history_of_atom.html).
When Becquerel abandoned this work, it Pierre and Marie Curie picked up the slack and went on to discover other radioactive elements. Marie suggested that the uranium, and the new elements, were somehow dissolving over time and sending off radiation known as "radioactivity". For the first time it was thought that atoms might be composed of even smaller particles that could be further researched.
J.J. Thomson: At about the same time as radioactivity was being found, J.J. Thomson and others were performing experiments with cathode ray tubes. When a potential is placed between the cathode (the negatively charged plate) and the anode (the positively charged plate) a "ray" of electric current passes from one plate to the other. Thomson discovered the ray was composed of particles. (http://www.neoam.cc.ok.us/~rjones/Pages/online1014/chemistry/chapter_8/pages/history_of_atom.html)
In later experiments he predicted what metal was used to make the electrodes and what gas was used to filled the tube. Each time, the properties of the ray particles were the same. He concluded that negatively charged particles were subatomic particles which were part of every atom. He also concluded there were neutral and positive charges. Based on these conclusions Thomson proposed that an atom was composed of a ball of positive charges with "corpuscles" of negative charge inside it(http://www.neoam.cc.ok.us/~rjones/Pages/online1014/chemistry/chapter_8/pages/history_of_atom.html). The corpuscles were later known as electrons.
Ernest Rutherford: Thomson had discovered that the atom was composed of positive and negative charges and that the mass was made up of these. If the negative and positive charges left empty space in the atom, particles that are shot at the atom might be able to pass through them. In 1909, with the help of Hans Geiger and Ernest Marsden, Rutherford worked on an experiment. The gold tin foil experiment. This later led to the discovery of the size of the atom and the nucleus.
Henry Moseley: When Henry Moseley tried to determine the frequency of an x-ray in an experiment, he found as a result a pattern he called the “Q” value. This led him to believe that there was a quanity that increased as he went from one element to the next. This value would later become known as the atomic number.
James Chadwick: Only two known elementary particles had been identified, the proton and the electron, until James Chadwick. Protons had a mass of 1 and a charge of +1, while electrons had no mass and a charge of -1. There was still a problem though, there was extra mass! The problem of the extra mass was solved in 1932 when Chadwick identified the neutron. While studying the radiation resulting from the combining of beryllium with alpha particles, Chadwick found a particle with about the same mass as a proton being let off. This was known as the neutron.
****when i cited words i couldnt re-word, since is was the actually experiment, i used the source at the very end instead of at the end of every single tiny thing.
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